The nucleic acids like DNA and RNA are composed of nitrogenous bases, sugar and phosphorous. These nitrogenous bases are adenine (A), guanine (G), thymine (T), and cytosine (C). These nitrogenous bases have hydrogen bond between opposing DNA strands to form the rungs of the "twisted ladder" or double helix of DNA or a biological catalyst that is found in the nucleotides. Adenine is always paired with thymine, and guanine is always paired with cytosine. These are known as base pairs. Uracil is only present in RNA, replacing thymine. Pyrimidines include uracil, thymine, cytosine. They have a single ring structure. Purines include adenine and guanine. They have a double ring structure.
